Endorectal ultrasound is the most reliable method in staging rectal cancer and is superior to computed tomography. In a prospective series of 152 consecutive patients, comparison of preoperative ultrasound staging and postoperative histopathological staging resulted in an overall accuracy of 90.1%. Overstaging was observed in 9.2%, understaging in 0.7%. Peritumoral inflammatory changes, preoperative radiotherapy and localization of the tumor in the lower rectum were the main reasons for overstaging. Understaging was seen in stenotic or only minimal invasive tumors. Accuracy in staging lymph nodes was 78.5%. Inflammatory changes in the nodes were responsible for either overstaging or understaging. Whether diagnosis will be improved with a higher frequency--mainly in lymph nodes--has yet to be proved. With knowledge of the reasons for misinterpretation, endorectal sonography is--mainly in the lower rectum--a valuable aid in evaluation of surgical procedure.